Wave-transmission system



y 1930. R. c. MATHES WAVE TRANSMISSION SYSTEM 2 SheeE -Sheet 1 OriginalFiled March 13, 1925 May 6, 1930. R. c. 'MATHES WAVE TRANSMISSION SYSTEM2 sheets-sheet 2 RANGE asurms RANGE 55 unns Original Filed March 15,1925 obL r f w.

E. 2 5.3.50 mommmmmtou COMPRESSOR lNPUT IN TU hue/71hr. @oberfCMa/fies vPatented May 6, 1930 v UNITED. STATES PATENT OFFICE ROBERT C. MATHES, OFWYOMING, NEW JERSEY, ASSIGNOR, BY MESN E .ASSIG-NIIYIIEINTS, TO WESTERNELECTRIC COMPAN Y, INCORPORATED, A CORPORATION OF NEW YORKwavn-rnansmrssron SYTEM Application filed March 13, 1925, Serial No.15,227. Renewed October 17, 1929.

This invention relates to wave transmission systems and particularly tothe control of the energy level of transmission in such systems.

An object of the invention is to provide methods of and means foreflectively transmitting energy having a wide range of volumes oversystems inherently capable of transmitting only a narrow rangeofvolumes.

In the transmission of music or,voice hav ing a wide variation involume, such as is obtained in connection with the broadcasting oforchestra and public address programs, over transmission systems, suchas telephone lines or radio signaling systems, it is desirable tomaintain the power level between definite limits to avoid distortion ofthe transmission. An upper limit for the power level of the program tobe transmitted is necessary, for example, to avoid overloading lineapparatus, such as vacuum tube repeaters, and to avoid objectionablecross-talk, while a lower limit is necessary so that the noise in theapparatus used or inthe line will not be troublesome.

To illustrate, it has been determined that the upper power limit fortwo-wire repeated transmission lines is about +5 transmission units, andthe lower about -25 transmission units. A transmission unit is definedso that the number of transmission units corre- 'sponding to a powerratio of cuits, the upper limit may be about 10 transmission unlts. Therange, therefore, through which the power may vary is to transmissionunits. For high quality transmission, this range must be still furtherreduced, since the noise that can be permitted is less than that incommercial voice channels. The range given is sufiicient for speech,whose variation with time is ordinarily not greater than about 25transmission units. Orchestral music, however, generally has a volumerange of to 60 transmission units or more. Under such conditions it isnecessary to provide some means for maintaining the power within thelimits of a transmission system, when programs having such a wide rangeof volumes are to be transmitted over it.

is equal to In accordance with the present invention, control of thevolume of transmission is obtained by introducing at each end of thetransmission line a variable loss which is a function of power level andnot of frequency. Compression at the input end is obtained by causingthe loss to increase as the input level increases, and expansion at theoutput end is produced by a converse operation.

Specifically the control of the volume is accomplished by sending thetransmission through circuits having variable impedance elements, theimpedance of these elements being made a function of power level. Thevariable impedance element at each end of the line comprisesthree-electrode vacuum tubes, the impedance of these tubes being variedby the application of a control potential to their grid circuits. Thecontrol potential is obtained by rectifying some of the received energybefore being varied. By one method the compression or expansion isobtained by the shunting action of the variable impedances on the line.By another method the compression or expansion action is obtained byincorporating the variable impedance elements in bridge circuits at eachend of the line, and by utilizing their variation in impedance withpower level to change the balance of .the-

bridge circuits.

The various features and objects of the in-- vention will be understoodfrom the following detailed description when read in connection with theaccompanying drawings.

In the drawings: Fig. 1 is a diagrammatic view illustrating atransmission system embodying the shunt control modification of the-Fig. 1, a transmission system comprising a designed in accordance withthe brid coil 3 is also associated with'a volume.

control channel 11, which comprises a high pass filter 12, a suitablerectifier 13, a low pass filter 14 and a variable impedance elementcomprising thethree-electrode space discharge devices 15 and 16, whichare connected in push-pull relation at their outputs.

The output circuits of the space discharge devices 15 and 16 by means oftransformer 18 are connected effectively in shunt of the main channel 4between the filter 6 and the amplifier 7.

At the receiving station B of Fig. 1, the transmission line ML isassociated through hybrid coil arrangement 3 with a main channel 4,which through amplifier 5 and low pass filter 6 is connected to theinput circuit of an amplifier 7. The usual balancing impedance 10 isassociated with the hybrid coil 3 to balance the incoming line ML. Theoutput circuit of amplifier 7 may be connected to a suitable receivingapparatus R or to outgoing transmission lines. The transmission line MLat the receiving station through the hybrid coil arrangement 3' is alsoconnected to a volume control channel 11, similar to the channel 11 atthe'sending station, which comprises the high pass filter 12, therectifier 13, the low pass filter 14, and the variable impedance elementcomprising the threeelectrode vacuum tubes 15 and 16, which areconnected in push-pull relation at their outputs. The output circuits ofthe vacuum tubes 15 and 16 through transformer 18- are connectedeffectively in shunt of the main channel 4 betweenthe filter 6' and theamplifier 7 The purpose of the filter combination used at the sendingand receiving stations in the system of Fig. 1, and also in the systemof Fig. 2, and the particular features'of the individual filters of thecombination are de-- scribed in detail later. These filters may berinciples set forth in the patents to Camp ell, Nos. 1,227,113 and 1,227,114, dated Ma .22, 1917.

' The 0 eration of the system 0 Fig. 1 will now be escribed. Soundvariations, corresponding to the music or voice program to betransmitted to station B, are received by the microphone transmitter 1,'and the corresponding electrical variations produced in transmittercircuit-T are transmitted through transformer 2 and hybrid coil arranment 3 to main channel 4, and after ampli cation in the amplifier 5 passthrough the low pass filter 6. Aportion of the energy received by thehybrid coil 3, however, is tapped off 'resistance 17, so that arectified control potential is applied to the two tubes symmetrically.The rectified control potential causes like variations in the gridvoltage of each tube and consequently equal variations in the plateimpedances of the two tubes. As the output circuit of the variableimpedance device comprising push-pull tubes 15 and 16 is coupled bymeans of transformerlS across main channel 4, a loss corresponding tothe plate impedances of tubes 15 and 16 in series is thus effectivelyinserted in shunt to that channel between the filter 6 and the amplifier7. The push-pull arrangement of the tubes making up the variableimpedance element provides that, while their impedances are responsiveto changes in the control potential applied to their ids, their combinedimpedance as viewed rom the main transmission line is substantiallyindependent of the magnitude of energy passing over that line. Thehybrid coil arrangement 3 provides a means whereby the variableimpedance element will not react on the amount of input supplied to therectifier 13, even if amplifier 5 were omitted.

As the impedances of the tubes 15 and 16 in the manner describedv aboveare controlled by the power level of the energy input to the system, itis readily seen that the loss in shunt to the channel 4 will also varyin accordance with that input level. Also, it is readily seen that, ifthe poling of the grids of the tubes '15 and 16 with respect to therectifier 13 is properly selected, the impedances of these tubes willdecrease, and therefore the loss inserted in channel 4 will increase.With increase in the input level and decrease with decrease in theinput level. If the volume control apparatus is properly designed, theloss varying with input level in channel 4 will be such as to -compressthe volume range of the variations passed by filter. 6 within therangevcapable of being transmitted satisfactorily by the transmissionsystem. The volume range of the variations to be transmitted over theline ML being controlled in this manner, it may be necessary for sat- 7through the amp isfactory transmission to raise or lower the averageamplitudelevel, and this is accomplished by sendin the. compressedvolume er 7 before transmitting it over the transmission line ML.

At station B,the variations received from the line ML aretransmittedthrough the hybrid coil 3 to the main channel 4, and after amplificationin the amplifier 5 ass through low pass filter 6'. A portion 0 theenergy received by the hybrid .coil 3', however, is tapped off at theoints 8' and 9" of hybrid coil '3 and is supp ed to high pass filter. 12in the volume control channel 11". The variations in the output offilter 12" are rectified b the rectifier 13 and after passing through te low pass filter 14' are supplied to the input circuit of push-pulltubes 15 and 16' across the resistance 17 in the common branch of theirgrid circuits, to vary the impedance of the two, tubes 15 and 16 inaccordance with the input power level invthe manner described inconnection with the volume control channel '11 at the sending station.As the output circuits of the tubes 15- and 16" by means of transformer18' are connected in shunt of the main channel 4, a

- loss corresponding to the impedance of the tubes is thus inserted inshunt to channel 4. .The poling of the grids of the tubes 15 and 16"withrespect to the rectifier 13" is made the'reverse from that of thesimilarly designated elements at the sending station, so thatin thiscase, the impedance of 'the tubes '15 and 16'. will increase, andtherefore the loss in shunt to the main channel 4 between filter 6' andthe input circuit of amplifier 7 will decrease with increase in theinput level of the transmitted variations and increase with decrease inthe input level. This -loss will have the effect of expanding the volumerange of the variationsreceived from the output circuit of filter 6' inchannel 4. By suitabledesign of the apparatus at .the tubes 15 andlfi'froln reactin each station, the net e uivalent of the circuit ismade independent 0 amplitude (or power level), and the volume range ofthe variations supplied to the amplifier? willi'be an exact copy of theinput to. the compression apparatus at the sending station. As {atstation A, the hybrid coil arian e men't 3' is provided to prevent thevaria leimpedances of on the amount of input supplied to the rectlfier13'. The average amplitudelevel'of the received variations are changedin the amplifier 7' to any desired level, and the" variations suppliedto the receiver R,,or if desired sent out over another transmission lineto another station.

- I The push-pull arrangement of the vacuum tubes 15,, 16 and 15, 16' inthe volume control channels at the sendingand receiving stationsrespectively, also has two other advantages:

in the first place, the effect of harmonics caused by the insertion ofavvariable imped ance in the main channel is negligible in such anarrangement; secondly, thorough rectification of the control current isunnecessary as troublesome A. C. components of the sending and receivingstations will represent an envelope of the received volume (or theaverage received volume over any desired range of time), it is necessaryto filter out the speech frequency components in the rectified current.In so doing, however, a time lag will beintroduced into the controlcurrent so that the controlof volume will no longer be simultaneous withvolume changes in the received energy. Hence, it may be necessary tointroduce delay circuits, such as filters, in the main transmission pathto compensate for these differences in time.

The filters 14, 14' in volume control channels 11, 11- respectively arelow pass filters designed to remove the audio frequency components fromthe rectified currents which control the impedances-of the push-pullcir- 1 cuits. The filters 6, 6 in the main channels 4, 4, respectively,are provided to put the necessary time delay in the transmission. IThese filters are designed to pass all of the audio frequency componentsessential to the propernumber of sections in the'filters 14, 14" by aThe number of filter sections in the factor equal to the ratio of theirrespective cut-01f frequencies.

As the cut-ofl frequencies of filters 14, 14. should ordinarily be quitelow,.thiswould mean that filters havin a rather large number of sectionswould lie needed-to put the c necessary compensating time lags into thevaried transmission. It is, therefore, desirableto put the cut-oflfrequencies of filters 14, 14' as high as possible. This can be donesince the envelope of the frequency components above some specifiedvalue can fairly represent the volume changes of the total energy. Hi hpass filters 12,12 can then be inserted in rout of the rectifiers 13,13' in volume control'channels 11, 11' respectively.

The cut-off frequencies'of the low pass filters 14,14 can then be placedalittle below the cut-off frequencies of the filters 12, 12' instead ofbelow the total range of frequencies involved. As stated above, thefilters used may be designed in accordance with the principles set forthin the patents to Campbell. Nos. 1,227,113 and 1,227,114, dated May22,."

In Fig. 2 is illustrated the bridge control modification of theinvention. The system shown in Fig. 2 differs from the system of coil 3as in the previous figure, is connecteddirectly to the main channel 4.The main channel 4, instead of being connected through amplifier 5 andfilter 6 to the input circuit of amplifier 7 as in the previous figure,is connected-to filter 6, the output of which'is coupled by means oftransformer 20 to the input circuit of amplifier 21, the output circuitof which is associated with-the input circuit of amplifier 7 through thehybrid coil arrangement 22. The terminals of the secondary of the outputtransformer 18, instead of being connected directly across the mainchannel 4 as in the previous figure, are connected through the linecoils of thehybrid coil arrangement 22 to the terminals of a balancingimpedance 23. The input circuit of the variable impedance devicecomprising the vacuum tubes 15' and 16' at stationB, instead of beingconnected through filter 14', rectifier 13', filter 12' and hybrid coil3 across the line ML as in the system of Fig. 1, is connected directlythrough a pilot channel 24 to the output of filter 14 in the volumecontrol channel 11 at station A. The transmission line ML instead ofbeing connected at station B through hybrid coil arrangement 3. andthrough amplifier 5 and filter 6 directly to the input circuit ofamplifier 7 as in the system of Fig. 1 is connected directly to theinput of filter 6 in the main channel 4, and the output of filter 6' isassociated with the input circuit of amplifier? through amplifier 21 andthe hybrld coil arrangement 22'. The terminals of the secondary windingof output transformer 18, instead of being connected across channel 4'as in Fig. 1, are connected through the line coils of the hybrid coilarrangement 22 to the terminals of a balancing impedance 23'. The otherelements of the system of Fig. 2 correspond to the similarly designatedelements in Fig. 1.

The operation of the system of Fig. 2 will now be described. At stationA, sound variations corresponding to the music or s eech program to betransmitted -to station are received by the microphone transmitter 1,and the. corresponding electrical variations in the transmitter circuitT are transmitted through transformer 2 to the input, of filter 6 inmain channel 4. The filter 6 passes all the audio frequency componentsof the received variations which are essential to the propertransmission of music or speech,

the transmitted variations being supplied through transformer 20 to theinput circuit of amplifier 21. The corresponding amplified; variationsin the output circuit of amplifier 21are impressed on the hybrid coilarrangement 22. A ergy in the output circuit'of transformer 2 throughtransformer 19 is supplied to the input of high pass filter 12 in thevolume control channel 11. The transmitted enorgy in the output offilter12 is rectified by the rectifier 13 and t which removes the audiofrequency components from the rectified current. The rectifiedvariations in the output of filter 14 are applied to the resistance 17in the common branch of the grid circuits of the push-pull tubes 15 and16, and in the manner described in connection with the correspondingcircuit in Fig. 1, control the plate impedances of the two tubes 15 and16 in accordance with the changes in the power level of the energygenerated by transmitter 1. As in the case of the corresponding elementsof Fig. 1, the polarity of t e grids of the tubes 15 and 16 with respectto the am lifier 13 is made such that the impedance 0 these tubes willincrease with lncrease in the power level at the input and decrease withdecrease in the power level at the input.

It is readily seen, that the arrangement of the circuits as shown issuch as to form a Wheatstone bridge circuit comprising the coils of, thehybrid coil 22, the constant balancing impedance 23, and the variableimpedance looking towards the transformer 18, which connects the outputof the tubes 15 and 16 to the hybrid coil arrangement 22. Designatingthe impedance looking towards transformer 18 as Z and thelconstantimpedance of 23 as Z, it is readily seen that the assed to the filter1e,

part ofthe received enloss through the hybrid coil 22 depends upon therelation of Z to Z. If Z is less than Z,

then it should be nearly equal to Z with high level input and decreasein impedance as the.

input level becomes lower, so that the .un-

balance will be eater and the loss through the bridge circu1tless.Because of the poling of 'the input to the push-pull set of tubes 15and-16, this loss will'be low for low level inputs to the system andhigh for high level inputs. Therefore,-the speech or music va-' riationsfrom the output circuit of amplifier 21 when transmitted through hybridcoil 22 The amount will be compressed in volume. of compression can befixed at a desired value by selecting the apparatus constants to obtainthe required ratio of Z to Z. This ,may

of compresresistance to the plate impedances of the adding resistance inseries with the secondary winding of transformer 18. i

The compressed volume transmitted through the hybrid coil 22 is suppliedto the input circuit of amplifier 7. The amplifier 7 may be utilized tochange the average amplitude level of the compressed variations to anydesired value before transmitting them'over the line ML.

In the hybrid coil type of compressor just described, it is possible ofcourse to replace the constant balancing impedance 23 with anotherpush-pull set of tubes so poled that as the impedance of one setincreases the impedance of the other decreases. However, satisfactoryoperation for most purposes may be obtained by using a constantbalancing impedance instead of the double variation arrangement.

At station B, the transmittedvariations after passing through the lowpass filter 6 are amplified by the amplifier 21 and impressed on thehybrid coil 22. To control the volume of the transmitted variations atstation B, a pilot channel 24 connected across the output of the filter14 in the channel 11 at station A, is utilized to convey rectifiedcontrol current, which will be proportional to the original input atthetransmitter 1, to the input cireuitof the variable impedance devicecomprising the tubes 15' and 16' at station E. This control current isapplied to the resistance 17 in the common branch of the grid circuitsof tubes 15' and 16, and in the manner described in connection with thecorresponding elements at the sending station, will cause the plateimpedances of the tubes to vary in accordance with changes in the powerlevel of the energy generated by transmitter 1 at the sending station.However, in this case, the poling of the grids of the tubes 15 and 16'with respect to the poling of the rectifier 13 is reversed from thatused at the sending station, so that the plate impedancesof the tubes15' and 16' will decrease with increase in the input power level andincrease with decrease in the input level.

As the terminals of the secondary winding of the transformer 18,associated with the output circuits of the tubes 15' and 16' areconnected through the line coils of hybrid coil 22' across the balancingimpedance 23, a Wheat-stone bridge circuit is formed comprising the linecoils of hybrid coil arrangement 22, the constant balancing impedance23', and the impedance looking towards the transformer 18'. As in thecase of the similar arrangement at station A the loss through the hybridcoil 22 will be determined by the relation of the impedance lookingtoward transformer 18 to the balancing impedance 23'. In this case,because of the reverse poling between the rectifier 13 and the tubes 15'and 16, the loss through hybrid coil 22 will be low for high levelinputs and high for low level inputs. Therefore, the speech variationsfrom the output circuit of amplifier 21' transmitted through hybrid coil22' will be expanded in volume range. If as at the sending station, theconstants of the circuit are properly selected the volume range of thetransmitted variations may be made a copy of the input to thecompression apparatus at the sending station. As at the sending station,it may be necessary in some cases to add a constant resistance in serieswith the secondary winding of the output transformer 18' to get thedesired amount of expansion. The expanded variations are transmittedthrough the amplifier 7 to adjust the average amplitude to any desiredvalue and are received in suitable receiving apparatus R, or areretransmitted over other lines to other stations.

It is to be noted that, while variations in the pilot powercorrespond-to those in the input to the compression apparatus at stationA, the volume range that can be transmitted by the pilot channel 24 isnot as limited as that which can be transmitted by the main line MLbecause of the difierent circuit characteristics, provided this pilotpower is the output of the rectifier 13. The pilot wave being pulsingdirect current with a frequency spectrum smaller than that of the wavein the main transmission line, the frequency band required for itstransmission may be much less than that required for the transmission ofmusic. By the use of a pilot channel, therefore, the effect of linenoise on the volume control at the receiving station can besubstantially eliminated.

' Theoperation of thecircuits of Fig. 1 or 2 may be .more clearlyunderstood from a consideration" of the transmission level diagram ofFig. 3. It will be assumed for the purpose of explanation that thevolume of the variations, corresponding to the speech or music to betransmitted, in the output of filter 6 in the system of Fig. 1, or inthe output circuit of the amplifier 21 in the system of Fig. 2, variesfrom a maximum value of 5 transmission units to a minimum value of 4.5transmission units. It will be also assumed that the maximum range ofvolume that can be efiiciently transmitted by the transmission line ofFig. 1, or of Fig. 2, is 20 transmission units. In Fig. 3, changes inthe maximum volume or power of the energy in transmission from station Ato station B are shown in the upper curve, and changes in the minimumvolume or power of the energy in transmission from station A to stationE are shown in the lower curve. The maximum volume of the energy isfirst reduced by the volume controlling apparatus at station A to atransmission level of -25 transmission units, as indicated and is thenamplified by amplifier 7 to a transmission level of +5 transmissionunits. The maximum volume is then attenuated in passing over thetransmission line ML due to the line loss so that it drops down to 0transmis sion units. At station B, the volume controlling -apparatusreduces the maximum volume level to 5 transmission units, whichcorresponds to the original maximum volume in the output of filter 6 ofFig. 1, or in the output circuit of amplifier 21 of Fig. 2. The maximumvolume is then amplified by amplifier 7 to a transmission level of +20transmission units.

Similarly, the minimum volume of -4() transmission units in the outputof filter 6 of Fig. 1, or in the output circuit of amplifier 21 of Fig.2, at station A is first reduced by the volume control apparatus at thatstation to a level of -45 transmission units. The minimum volume is thenamplified by the amplifier 7 to a volume level of 15 transmission units.In transmission over the transmission line ML, the minimum volume isattenuated due to the line loss so that it falls to a level of 20transmission units. The minimum volume is further reduced by the volumecontrol apparatus at station B so that it drops to a level oftransmission units which corresponds to the original minimum volume atstation A. The minimum volume is then amplified by the amplifier 7' toraise the level to 15 tranmsission units.

It will be noted by reference to the curves that, at all correspondingpoints in the system between the I output terminals of the volumecontrol apparatus at station A and the input terminalsof the volumecontrol apparatus at station B, the maximum and minimum values forvolume level will be separated by a range of 20 transmission units,which is the range which can be carried satisfactorily by thetransmission line, and that at all corresponding points beyond theoutput terminals of the volume control apparatus at station B, themaximum and minimum values forvolume level are separated b a range of 35transmission units, which is equal to the range between the maximumvolume and the minimum volume at the input to the volume controlapparatus at station A. The transmitted music or voice programs whenreproduced at the out Figs. 1 and 2. The curves show variations in theoutput of the volume control apparatus with variations in inputindicated in to the TU input, that is, the case when there is nocompression in volume. Curve B represents the case where the adjustmentof the volume control apparatus is such that the output is less than theinput, but increases with mcrease in input at a gradually diminishingrate. The curve C represents the case where the adjustment is such thatthe output increases with increase in input at a gradual diminishingrate up to a maximum point, and falls off greatly with further increasein input giving a drooping characteristic. The important differencebetween the curves B and C to be noted, is that for compression of thetype shown in curve C, the same amount of output will be obtained fortwo different values of input, as indicated by the points X and Y on thecurve C, and that for the type of compression shown in curve B, eachvalue of output corresponds to only one input as indicated by the pointZ on curve B. In the type of compression shown in curve B, then, it isevident that the compressed output received over the line from thesending station may be used to control apparatus to cause its ownexpansion, as in the system shown in Fig.1. For the type ofcompressionshown in curve C, however, it is evident that some means, such as apilot channel, must be used to convey to the receiving station powerproportional to the original input, which power may be used to controlthe apparatus to cause the expansion of the compressed power receivedover the line. A system of that type has been illustrated in Fig. 2.

Only one stage of compression and one stage of expansion have been shownin the system of Fig. 1 and in the system of Fig. 2.

to 0 TU when the input TU is maximum;.

and it has been found that such total compression can easily be done inone operation. With the type of compression shown in curve B of Fig. 2,however, as many stages of compression may be used as desired, the onlyli'mit being a final output independent of input when the number ofstages is infinite.

The applications illustrated and described should be considered merelyas typical and not as limiting the invention, the scope of which isdefined in the appended claims.

What is claimed is:

1. The method of controlling the volume of energy passing over a system,which comprises rectifying a portion of the energ 1 input to the system,and continuously changing the volume in response to changes in therectified energy below a preassigned frequency, while preventing morerapid changes in the rectified energy from afi'ecting the volume.

2. In .a transmission system, a source of energy of varying volume, atransmission medium supplied with said energy, means to rectify aportion of the energy input to the system, and means responsive only'tochanges in the rectified energy below a preassigned fre quency tocontinuously control the volume of energy transmitted overt-he system.

3. In a signaling system, means to generate signaling energy havingvariations in volume, a transmission medium upon which said energy isimpressed, a transmission element in said medium, and means operating tomake the transmission efficiency of said transmission element a functionof the average volume over a limited period of time of the energyimpressed on said medium to control the volume of energy passing overthe system, said last mentioned means operating with all of itsconstituent parts in a fixed relation to one another.

4. In a signaling system, means to generate energy having variations involume, a transmission medium, means to control the volume of energypassing over the system and means for impressing said energy on saidmedium independent of said control means, said control means comprisingatransmission element whose impedance is a function of the averagevolume of the energy impressed on said medium, over a limited period oftime.

5. In a signaling system, means to generate waves of electrical energyin the speech or mus1c frequency range havlng variations in volume, atransmission medium upon which said energy is impressed, and means tocontrol the volume of energy transmitted over said medium, saidcontrolling means comprising a variable impedance element including aspace discharge device eflectively connectedto said medium. and meansfor controlling the impedance of said space discharge device inaccordance with the average volume otthe energy impressed on said mediumover a limited period of time.

6. In a signaling system, means to generate waves of electrical energyin the speech or music frequency range having variations in volume, atransmission medium, a space discharge device connected to said medium,and means for impressing the speech or music waves on said mediumindependent of said device, the impedance of said device determining thevolume of energy transmitted over said medium, and means to control theimpedance of said device in accordance with i the volume level of thespeech or music frequency waves impressed on said medium.

7. In a signaling system, means to generate signaling currents havingvariations in amplitude, a transmission circuit, a Wheatstone element,means to impress said signaling currents on said transmission circuitthrough the volume of energy transmitted over said medium comprising athree-electrode space discharge device having a grid circuit and anoutput circuit, said grid and output. circuits being associated inenergy transfer relation with said medium, means to produce a controlcurrent varying in accordance with the volume level of the energy fromsaid source, and means to apply said control current to said gridcircuit to control the impedance of said device.

9. In a transmission system, a source of signaling currents, atransmission medium. a Wheatstone bridge circuit comprising a variable'impedance element, means to impress said signaling currents on saidtransmission medium through said bridge circuit, and means to controlthe amount of unbalance current transmitted through said bridge circuitto said medium, the controlling means comprising means for varying theimpedance of said variable impedance device in accordance with the powerlevel of the signaling currents from said source.

10. In a transmission system, means togeni crate waves of electricalenergy in the speech or music frequency range, a transmission linesupplied with said energy,a three-electrode space discharge devicehaving a grid circuit and an output circuit, said output circuit beingconnected to said line, means also connected to said line to rectify aportion of the generated signaling energy, means to filter outvariations in the rectified energy above a pie-assigned frequency, andmeans to impress the filteredrectified energy upon said grid circuit tocontrol the impedance of said space discharge device, therebycontrolling the volume of energy transmitted over said transmissionline.

11. In a signaling system, a source of electrical energy representingspeech or muslc from said source, means to utilize the rectified energyto control the impedance of said transmission element in accordance withchanges in the power level of the energy from said source, and means forassociating said circuits in energy transfer relation with said mediumto prevent the variable impedance of said transmission element from re-,coil arrangement, said constant impedance element and said variableimpedance element being arranged with respect to each other to form aWheatstone bridge circuit, means to vary the impedance of said variableimpedance element directly in accordance with changes in the power levelof the energy from said source, thereby varying the balance of saidbridge circuit, a receiving circuit at the output end of saidtransmission medium, a second hybrid coil arrangement for impressing theenergy received over said medium upon said receiving circuit, a secondconstant impedance element, a second variable impedance element, saidsecond hybrid coil arrangement, said second constant impedance elementand said variable impedance element being so arranged with respect toeach other as to form a second Wheatstone bridge circuit, and means tovary the impedance of said second variable impedance element inverselyin accordance with changes in the power level of the energy from saidsource.

13. In a transmission system, a source of energy of varying levels, atransmission line, a hybrid coil arrangement at the input end of saidline, means to impress energy from said source on said line through saidhybrid coil arrangement, a constant impedance element, a space dischargedevice having an input circuit and an output circuit, said hybrid coilarrangement,-said constant impedance element and the output circuit ofsaid space discharge device being arranged to form a Wheatstone bridgecircuit, means to rectify a portion of the energy from said source,means to apply the rectified energytothe input circuit of said spacedischarge device so as to vary the impedance of said device directly inaccordance with changes in the power level of the energy from saidsource, thereby varying the balance of said bridge circuit, a receivingcircuit at the receiving end of said line, a second hybrid coil arrangement for impressing the energy received over said line on said'receivingcircuit, a second constant impedance element, a second space dischargedevice having input and output circuits, said second hybrid coilarrangement, said second constant impedance element and the outputcircuit of said second space dis,- charge device being arranged to forma second WVheatstone bridge circuit, means to convey a portion of saidrectified energy produced at the input end of said line to the outputend of said line, and means to apply the conveyed rectified energy tothe input circuit of said second space discharge device so as to varythe impedance of said second device in versely in accordance Withchangesin the power level of the energy from said source.

14. In a signaling system, a source of signaling energy, a transmissionmedium, an amplifier having an input circuit and an out put circuit atthe transmitting end of said medium, a hybrid coil arrangement, avariable impedance element, a constant impedance element, said hybridcoil arrangement, said variable impedance element, and said constantimpedance element being arranged to form a Wheatstone bridge circuit,means to impress signaling energy from said source upon the inputcircuit of said amplifier through said hybrid coil arrangement, and tovary the impedance of said variable impedance element in said Wheatstonebridge circuit in accordance with changes in the power level oftheenergy from said source to change the balance of said Wheatstonebridge circuit, thereby controlling the loss through said hybrid coilarrangement, and means to impress the energy in the output circuit ofsaid amplifier upon said medium.

15. In a transmission system, a source of energy, a transmission medium,an amplifier having input and output circuits at the transmitting end ofsaid medium, said output circuit being connected to said medium, aWheatstone bridge circuit comprising a hybrid coil arrangement, avariable, impedance element and a constant impedance element, means toimpress energy from said source on the input circuit of said amplifierthrough said hybrid coil arrangement, means to vary the impedance ofsaid variable impedance element in said Wheatstone'bridge circuit inaccordance with changes in the power level of the energy from saidsource so as to compress the amplitude range of the energy impressed onsaid medium through said hybrid coil arrangement and said amplifier, asecond amplifier comprising input and output circuits at the receivingend of said medium, a receiving circuit connected to the output circuitof said second amplifier, a second Wheatstone bridge arrangementcomprising a second hybrid coil arrangement, a second constant impedanceelement and a second variable impedance element, meansto impress theenergy received over said medipulse um on the input circuit of saidsecond amplifier through said second hybrid coil arrangement, means totransmit energy from said source to the receiving end of said medium,means to apply the transmitted energy to said second variable impedanceelement to vary its impedance inversely in accordance with changes inpower level of the energy from said source, thereby expanding theamplitude range of the energy received by said receiving circuit throughsaid second hybrid coil and said second amplifier.

16. In a signaling system, a sourceof signaling energy of varyingvolume, a transmission circuit comprising a main channel and anauxiliary channel, a high pass filter in said auxiliary channel, meansto impress a portion of said signaling energy upon said auxiliarychannel through said high pass filter, means to rectify the energytransmitted by said high pass filter, a low passfilter as sociated withthe rectifyin means and adapted to transmit the recti ed energy below apreassigned frequency, a volume con trol device in said main channelresponsive to the energy transmitted by said low ass filter means toimpress signaling energy om said source upon said volume control devicethrough said main channel, and means to load" said main channel withinductance to synchronize the operation of said volume control devicewith volume changes in the energy impressed on said device through saidmain channel.

17. In a signaling system, a source of electrical energy representingspeech or music having varlations in volume, a transmission mediumsupplied with said energy, a space discharge device connected to saidmedium, means alsoconnected to said medium for rectifying a portion ofthe energy from said source, means to produce a voltage from therectified energy and apply the voltage to the device to control the imedance of said device in accordance with c anges in the power level ofthe energy from said source, means to associate said device with saidmedium to introduce the impedance. of said device into said medium, andmeans to prevent the variable impedance of said device from reacting onthe amount of input supplied to said rectifying means. 7

18. In a transmission system, a source of signal impulses to betransmitted, means to impress said impulses on the system, a branchcircuit including a space discharge device, means in said branch circuitfor' controlling the'imlpedance of said device in accordance with t eaverage volume of the signal impulses over limited time periods, andmeans for efiectively introducing the impedance of i said device intosaid transmission system to control the volume of the transmitted im-19. In a transmission system, a source of speech ener of varying volume,a transmission me 'um supp ed. with said ener means to rectify a portionof the ener mput to the system, means responsive o y to changes in therectifiedenergy below a preassigned frequency to continuously controlthe volume of energy transmitted over the stem, and delay means in saidtransmis-. sionmedium between the point therein where the volume of thetransmitted energy is controlled and said source for compensating forthe lag in operation of said control means.

speech or music frequency range, atransmission line supplied with saidenergy a threeelectrode space discharge device having a grid'circuit andan output circuit, said GRIP]? circuit being connected to said line ando ,fering an impedance to the energy transmitted thereover, means alsoconnected to said line for rectifying a portion of the generatedsignaling energy, means for filtering out variations in the rectifiedenergy above a preassigned fre uency, means to impress the filteredrecti ed energy upon said id circuit to control the impedance of sa dspace discharge device, and dela means in the transmission line betweent e connection of said output circuit thereto and the wave generatingmeans to compensate for the la in the control of said' impedanceintroduced by said filtering means.

In witness whereof, I hereunto subscribe my name this 12th day of March,A. D. 1925.

ROBERT C. MATHES.

